Compressing Pixel Data

This tutorial is about compressing a dataset’s Pixel Data and covers

  • An introduction to compression

  • Using data compressed by third-party packages

  • Compressing data using pydicom

It’s assumed that you’re already familiar with the dataset basics.


This tutorial uses packages in addition to pydicom that are not installed by default, but are required for RLE Lossless and JPEG-LS Near-lossless compression of Pixel Data. For more information on what packages are available to compress a given transfer syntax see the image compression guide.

Installing using pip:

python -m pip install -U pydicom numpy pylibjpeg pyjpegls

Installing on conda:

conda install numpy
conda install -c conda-forge pydicom
pip install pylibjpeg pyjpegls


DICOM conformant applications are usually required to support the Little Endian Implicit VR transfer syntax, which is an uncompressed (native) transfer syntax. This means that datasets using Little Endian Implicit VR have no compression of their Pixel Data. So if applications are required to support it, why do we need Pixel Data compression?

The answer, of course, is file size. A CT Image instance typically consists of 1024 x 1024 16-bit pixels, and a CT scan may have hundreds of instances, giving a total series size of hundreds of megabytes. When you factor in other SOP Classes such as Whole Slide Microscopy which uses even larger full color images, the size of the uncompressed Pixel Data may get into the gigabyte territory. Being able to compress these images can result in significantly reduced file sizes.

pydicom can perform RLE Lossless compression without needing to install any additional packages, and can perform JPEG-LS Lossless and JPEG-LS Near-lossless compression with the installation of pyjpegls. For all other transfer syntaxes it’s entirely up to you to compress the Pixel Data in a manner conformant to the requirements of the DICOM Standard.


We recommend that you use GDCM for Pixel Data compression as it provides support for all the most commonly used Transfer Syntaxes and being another DICOM library, should do so in a conformant manner.

The general requirements for compressed Pixel Data in the DICOM Standard are:

  • Each frame of pixel data must be encoded separately

  • All the encoded frames must then be encapsulated.

  • When the amount of encoded frame data is very large then it’s recommended (but not required) that an extended offset table also be included with the dataset

Each Transfer Syntax has it’s own specific requirements, found in Part 5 of the DICOM Standard.

Encapsulating data compressed by third-party packages

Once you’ve used a third-party package to compress the Pixel Data, pydicom can be used to encapsulate and add it to the dataset, with either the encapsulate() or encapsulate_extended() functions:

from typing import List, Tuple

from pydicom import examples
from pydicom.encaps import encapsulate, encapsulate_extended
from pydicom.uid import JPEG2000Lossless

# Fetch an example dataset
ds = examples.ct

# Use third-party package to compress
# Let's assume it compresses to JPEG 2000 (lossless)
frames: List[bytes] = third_party_compression_func(...)

# Set the *Transfer Syntax UID* appropriately
ds.file_meta.TransferSyntaxUID = JPEG2000Lossless
# For *Samples per Pixel* 1 the *Photometric Interpretation* is unchanged

# Basic encapsulation
ds.PixelData = encapsulate(frames)

# Extended encapsulation
result: Tuple[bytes, bytes, bytes] = encapsulate_extended(frames)
ds.PixelData = result[0]
ds.ExtendedOffsetTable = result[1]
ds.ExtendedOffsetTableLength = result[2]

Compressing using pydicom

The RLE Lossless and JPEG-LS Lossless transfer syntaxes are currently supported for lossless compression of Pixel Data using pydicom. The easiest compression method is to pass the UID for one of those transfer syntaxes to Dataset.compress():

>>> from pydicom import examples
>>> from pydicom.uid import RLELossless
>>> ds = examples.ct
>>> ds.compress(RLELossless)
>>> ds.save_as("ct_rle_lossless.dcm")

This will compress the existing Pixel Data and update the Transfer Syntax UID before saving the dataset to file as ct_rle_lossless.dcm.

JPEG-LS Near-lossless is currently the only transfer syntax that supports lossy compression, which works similarly to lossless but requires passing one or more keyword parameters to control the image quality. In the case of JPEG-LS Near-lossless this means the jls_error parameter:

>>> from pydicom import examples
>>> from pydicom.uid import JPEGLSNearLossless
>>> ds = examples.ct
>>> ds.compress(JPEGLSNearLossless, jls_error=3)

This will compress the existing Pixel Data with an absolute error of no more than 3 pixel intensity units.


pydicom makes no recommendations for specifying the image quality for lossy encoding methods. Any examples of lossy encoding are for illustration purposes only.

Each supported compression method has a corresponding encoding guide that can be used to help you understand it’s requirements:

If you’re creating a dataset from scratch you can instead pass a ndarray to be compressed and used as the Pixel Data:

>>> import numpy as np
>>> arr = np.zeros((ds.Rows, ds.Columns), dtype='<i2')
>>> ds.compress(RLELossless, arr)

Note that the shape, dtype and contents of arr must match the corresponding elements in the dataset, such as Rows, Columns, Samples per Pixel, etc. If they don’t match you’ll get an exception:

>>> arr = np.zeros((ds.Rows, ds.Columns + 1), dtype='<i2')
>>> ds.compress(RLELossless, arr)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../pydicom/src/pydicom/", line 1957, in compress
    encoded = [f for f in frame_iterator]
  File ".../pydicom/pixels/encoders/", line 678, in iter_encode
  File ".../pydicom/pixels/encoders/", line 304, in validate
  File ".../pydicom/pixels/encoders/", line 333, in _validate_array
    raise ValueError(
  ValueError: Mismatch between the expected ndarray shape (128, 128) and the actual shape (128, 129)

A specific encoding plugin can be used by passing the plugin name via the encoding_plugin argument:

>>> ds.compress(RLELossless, encoding_plugin='pylibjpeg')

The plugins available for each encoder are listed in the API reference for the encoder type.

Implicitly changing the compression on an already compressed dataset is not currently supported, however it can still be done explicitly by decompressing prior to calling compress(). In the example below, a matching image data handler for the original transfer syntax - JPEG 2000 Lossless - is required.

>>> ds = examples.jpeg2k
>>> ds.SamplesPerPixel
>>> ds.PhotometricInterpretation
>>> ds.PhotometricInterpretation = "RGB"
>>> ds.compress(RLELossless)

Note that in this case we also needed to change the Photometric Interpretation, from the original value of 'YBR_RCT' when the dataset was using JPEG 2000 Lossless compression to 'RGB', which for this dataset will be the correct value after recompressing using RLE Lossless.